A typical conventional microwave distributed amplifier circuit consists of periodically spaced field-effect transistors (FETs) which are connected by electrically short, high impedance microstrip lines. The design of such conventional amplifiers is generally discussed in, for example, the article by J. B. Beyer, et al., "MESFET Distributed Amplifier Design Guidelines," IEEE Trans. Microwave Theory Tech, Vol. MTT-32, March 1984, pp. 268-275, which is incorporated herein by reference. A distributed amplifier may be qualitatively described as a set of artificial input and output transmission lines which are coupled by (FET) transconductances.
A schematic view of a typical conventional distributed amplifier having an arbitrary number of sections "n" is shown in FIG. 1. The amplifier has an input port 10, an output port 11, an input line 14 consisting of a series of high impedance microstrip artificial transmission line elements 15, an output line 17 composed of a series of high impedance microstrip artificial transmission line elements 18, an image matching port 20 on the output line having an image matching impedance and DC drain bias source 21 connected to ground, an image matching port 23 on the input line having an image matching impedance and DC gate bias source 24 connected to ground, and a series of field effect transistors (FETs) 26 connected in a common-source configuration. The drains of each FET are connected by microstrip lines 28 to junction nodes between the microstrips 18 of the output line, and the gates are connected by connecting lines 29 to junction nodes between the input line microstrips 15. The terminations 21 and 24 are standard and well known in the art, and various gate and drain biases may be used. See, e.g., the biasing and termination circuits shown in U.S. Pat. Nos. 4,486,719, 4,543,535 and 4,595,881. The conventional distributed amplifier of FIG. 1 and the present invention are illustrated utilizing microstrip transmission lines although, of course, lumped inductance and capacitance transmission lines may also be used.
Due to the symmetry of distributed amplifier circuits, such as the one shown in FIG. 1, half of the current from each active device (e.g., FET) propagates in the forward direction (toward port 4) and half in the reverse direction (toward port 3). Unlike the "in phase" currents travelling in the forward direction, those travelling in the reverse direction are "out of phase" with each other and thus destructively interfere to some extent. Because of this cancellation, the conventional distributed amplifier is inherently a directional circuit. The directional properties of a conventional distributed amplifier have been compared to that of an ideal circulator. 0. P. Leisten, et al., "Distributed Amplifiers As Duplexer-Low Crosstalk Bidirectional Elements In S-Band," Electronics Letters Vol. 24, No. 5, Mar. 3, 1988, pp. 264-265.
Directivity (D) for the distributed amplifier can be defined as the ratio of the reverse output power (P.sub.out) at port 3 to the forward power output (P.sub.out) at port 4. In dB this is given as: EQU D.ident.-10log (P.sub.out.sup.- /P.sub.out.sup.+)=-20 log (.vertline.S.sub.31 /S.sub.41 .vertline.)
The conventional distributed amplifier is typically found to have a directivity of between -10 to -20 dB over most of the frequency range of interest.